Sheet metal molding for motor vehicles and process for producing a sheet metal molding for motor vehicles

ABSTRACT

A motor vehicle sheet metal molding of the invention is produced by hot forming from a metal sheet composed of an aluminum alloy which cannot be precipitation hardened, which contains at least magnesium and optionally manganese in addition to aluminum as alloy component. The motor vehicle sheet metal molding after forming has, at least locally, degrees of deformation which are above the forming limit curve of the aluminum alloy at room temperature. To produce the motor vehicle sheet metal molding, the metal sheet is heated at least locally to a temperature in the range from 200° C. to 400° C. over a period of from 1 to 60 seconds. The heated metal sheet is subsequently placed in a forming tool of a forming press and formed to produce the motor vehicle sheet metal molding.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP2011/003822, filed Jul. 29, 2011, which designated the UnitedStates and has been published as International Publication No. WO2012/016667 A1 and which claims the priority of European PatentApplication, Serial No. 10008039.9, filed Aug. 2, 2010, pursuant to 35U.S.C. 119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a motor vehicle sheet metal molding made of anon precipitation-hardenable aluminum alloy and a method for producing amotor vehicle sheet metal molding.

It is known to produce highly stressed vehicle components made ofaluminum sheet metal, primarily using precipitation-hardenable alloys.The conventional production process is the forming of the aluminum sheetmetal of low strength with subsequent ageing for achieving increasedstrength.

In aluminum alloys in which no strength increase by heat treatment(natural or artificial ageing) can be achieved, an increase of strengthcan only be achieved via deformation. In order to also be able toproduce complex geometries from sheet metals of these alloys, these areusually deformed, like precipitation-hardenable alloys, in a soft state.This means that these non precipitation-hardenable aluminum alloys arefor example soft annealed beforehand.

A disadvantage is that pressed parts which are produced by deformationof these soft annealed non precipitation-hardenable aluminum sheetmetals only experience a significant increase in strength in regions ofhigh deformability, with the result that the potential for lightweightconstruction using moldings from economically favorable nonprecipitation-hardenable aluminum alloys is relatively low. This is alsothe reason why non precipitation-hardenable aluminum alloys arepredominantly used in the chassis area as thick-walled components. Nonprecipitation-hardenable aluminum alloys are characterized mostly byvery good corrosion resistance. In addition, they are often used in nonhigh performance components, however, without the focus on particularlightweight construction.

In order to satisfy the current and future demands for the weightoptimization of vehicles, the goal has to be to save weight in allcomponents. This also applies to components made of nonprecipitation-hardenable aluminum alloys. These aluminum alloys areavailable as sheet metal of higher or high strength which are producedby cold rolling or by cold rolling with targeted re-annealing. To date,however, no complex components can be produced from these commerciallyavailable semi finished parts, even though this would be economicallyinteresting because of the possible weight reduction and the materialsavings.

The invention is based on the object to provide a high-quality motorvehicle sheet metal molding made of aluminum alloys, and which hasimproved component properties, and a high strength and/or a higherductility at the same strength.

The invention is further based on the object, to provide a possibilityhow to produce a complex component of high strength can be produced byforming from sheet metals made of roll hardened nonprecipitation-hardenable aluminum alloys.

DETAILED DESCRIPTION OF THE INVENTION

The part of the object relating to the device is solved by a motorvehicle sheet metal molding made by a process including the steps ofheating at least portions of a sheet metal made of a nonprecipitation-hardenable aluminum alloy which contains aluminum andmagnesium, wherein the sheet metal is of a material state defined by atemper designation according to European norm EN515:1993 selected fromthe group consisting of H12, H14, H16, H18, H19, H22, H24, H26, H28,H32, H34, H36 and H38; and forming the sheet metal into the motorvehicle sheet molding, wherein at least regions of the motor vehiclesheet metal molding have a degree of deformation above a forming limitcurve of the sheet metal at room temperature.

The motor vehicle sheet metal molding is produced from a nonprecipitation-hardenable aluminum alloy which according to the Europeanstandard EN 515:1993 is in the material state H12, H14, H16, H18, H19,H22, H24, H26, H28, H32, H34, H36 or H38 and contains beside aluminum asalloy component at least magnesium and optionally manganese. The motorvehicle sheet metal molding is produced by hot forming, wherein at leastportions of the starting sheet metal are heated and the starting sheetmetal is subsequently formed into the motor vehicle sheet metal molding.The motor vehicle sheet metal molding has at least locally degrees ofdeformation, which are above the forming limit curve of the aluminumalloy or the starting sheet metal at room temperature. European StandardEN 515:1993 establishes temper designations for all forms of wroughtaluminum and aluminum alloys and for continuously cast aluminum andaluminum alloy drawing stock and strip intended to be wrought.

The forming limit curve is an important parameter with regard to theforming. Each material has its individual forming limit curve which isusually determined according to Nakajima or Maciniak by means of cuppingtests on sheet metal samples. The forming limit curve describes themaximal formability of a material. When the degree of forming exceedsthe forming limit values and lie above the forming limit curve, neckingsor cracks can form in the component. As a result of overextended regionswith impermissible decrease in material thickness, a component safety isnot guaranteed.

According to the invention, motor vehicle sheet metal moldings made fromsheet metals made of roll hardened aluminum alloys are provided whichcannot conventionally be produced. A motor vehicle sheet metal moldingaccording to the invention has at least locally degrees of deformationwhich cannot be produced from the starting material at room temperatureor in which primary and secondary deformations occur which the materialwould not be able to endure without formation of cracks in the coldstate or at room temperature. Conventionally produced, the deformationcapacity of the material at room temperature would be exceeded duringforming and a failure due to cracks would result. Such cracks do notoccur in the motor vehicle sheet metal molding according to theinvention.

The motor vehicle sheet metal molding according to the invention is madeof a non precipitation-hardenable aluminum alloy or a sheet metal madeof such an aluminum alloy. This sheet metal was formed into a complexthree dimensional motor vehicle sheet metal molding by forming. The hereat least locally implemented degrees of deformation are such that thecomponent cannot be produced cold for example from a sheet metal in thematerial state H111 according to standard EN 515. Only the hot formingperformed according to the invention enables invention enables theproduction of the motor vehicle sheet metal molding according to theinvention which is of high quality without material flaws or materialweakening having a high strength or a higher ductility at the samestrength than conventionally produced motor vehicle sheet metal moldingsfrom aluminum alloys.

In sheet metal forming, the forming limit curve (also referred to asforming limit diagram)—as previously described—has established itselffor describing the formability. The forming limit curve describes thefailure threshold at different forming and tension states. In a forminglimit curve, failure by necking or the occurrence of cracks is shownspecific for the respective material. These sites of failure aredetermined in various tests and the forming limit curve is thengenerated for the different materials by plotting the main formingdegree φ1 against the secondary forming degree φ2.

The forming limit curve allows determining the process limits in theforming of sheet metal materials and thus serves for assessing thedeformation properties of sheet metals. Principally, the distance of thedeformation measured on a formed component to the forming limit curve isa measure of the safety in the production of drawn parts. By means ofanalyzing the deformation and comparison with the forming limit curve, areliable assessment of the forming process of sheet metals thus occurs.The forming limit curve illustratively defines the quality of thematerial and thus supports the user in selecting the right material.This is where the invention comes into play. Within the framework of theinvention, it was recognized that a motor vehicle sheet metal moldingmade of an aluminum alloy of the claimed type can be produced forproducing components with high strength and/or ductility, wherein thismotor vehicle sheet metal molding has degrees of deformation whichcannot be realized conventionally. Thus, the motor vehicle sheet metalmolding is characterized in that it has at least locally degrees ofdeformation which are above the forming limit curve of the aluminumalloy or the starting sheet metal made of this aluminum alloy at roomtemperature.

In a motor vehicle sheet metal molding according to the invention, theyield strength R_(p0.2) compared to the starting sheet metal is reducedby maximally 30%. Further, the motor vehicle sheet metal molding has ayield strength R_(p0.2) which is at least 20% higher than the yieldstrength of a comparable motor vehicle sheet metal molding, which isformed from a sheet metal made of an aluminum alloy in the temperdesignation O/H111 according to the European standard EN 515:1993. StateO means soft annealed. State O can refer to products in which theproperties demanded for the soft annealed state are achieved by hotforming methods.

Temper designation H111 means annealed and slightly strain hardened bysubsequent work steps for example stretching or straightening (less thanH11).

As mentioned, the yield strength R_(p0.2) of a motor vehicle sheet metalmolding according to the invention is significantly increased by atleast 20% compared to a conventionally produced motor vehicle sheetmetal molding. A motor vehicle sheet metal molding made of a 1 mm thicksheet metal which conventionally has a yield strength R_(p0.2) of 110N/mm², has now a yield strength R_(p0.2) of 132 N/mm² or higher. In amotor vehicle sheet metal molding made from a 1 mm to 3 mm thick sheetmetal, a yield strength R_(p0.2) of 105 N/mm² is conventionallydemanded. A motor vehicle sheet metal molding of this wall thickness hasa yield strength R_(p0.2) of at least 126 N/mm². Motor vehicle sheetmetal moldings made from sheet metal with a wall thickness of greaterthan 3 mm conventionally should have a yield strength R_(p0.2) of 100N/mm². According to the invention, a motor vehicle sheet metal moldingof this wall thickness and made of the aluminum alloy as claimed has ayield strength R_(p0.2) of at least 120 N/mm².

The part of the object relating to the method is solved by a method forproducing an open sheet metal molding including the steps of providing asheet metal made of a roll hardened, non precipitation-hardenablealuminum alloy comprising aluminum and magnesium, wherein the sheetmetal is in a material state defined by a temper designation accordingto European standard EN515:1993 selected from the group consisting ofH12, H14, H16, H18, H19, H22, H24, H26, H28, H32, H34, H36 and H38;heating at least a portion of the sheet metal to a temperature in arange from 200° C. to 400° C. within a time period of 1 to 60 seconds;inserting the heated sheet metal into a forming tool of a forming die;forming the sheet metal into the sheet metal molding; and cooling thesheet metal molding to a temperature below 200° C. within 60 secondsafter the heating.

The method for producing a motor vehicle sheet metal molding accordingto the invention can be described as follows:

An important step of the invention is that the sheet metal (startingsheet metal) is heated to a temperature in the range between 200° C. and400° C., in particular less than 370° C., and the heating and theproduction by pressing of the component is performed within a timeperiod of 1 to 60 seconds, after which the heated sheet metal isinserted into a forming die and formed into the sheet metal molding.

Preferably, the sheet metal is then cooled to a temperature smaller than200° C. within 60 seconds after the heating, preferably by forming in atool, whose temperature is smaller than 200° C., or is cooled in adevice after the forming below 200° C.

In particular, the heated sheet metal is placed into a cold forming toolwherein a cold forming tool relates to an unheated or not externallyheated forming tool.

Within the framework of the invention, the formed sheet metal isactively cooled. Principally, however, a passive cooling is alsoconceivable in which the sheet metal or the formed sheet metal moldingis removed from the forming die and cooled under ambient conditionswithout additional cooling measures.

With the conventional heating methods to temperatures to about 300° C. alarge portion of the strength is lost during heating in the furnace.However, it has been shown that the re-crystallization as well as theregeneration of the structure depends on a temperature-time threshold.Therefore, it is provided to heat a roll hard nonprecipitation-hardenable aluminum sheet metal which according to theEuropean standard EN515:1993 whose disclosure is hereby incorporated byreference, is in the material state H12, H14, H16, H18, H19, H22, H24,H26, H28, H32, H34, H36 or H38 and as alloy component beside aluminumcontains at least magnesium and optionally manganese, at a very highspeed and then form it in a cold forming tool so fast that a predominantportion of the strength is retained in the entire component. Accordingto claim 2 the yield strength R_(p0.2) of the produced motor vehiclesheet metal molding compared to the starting sheet metal is reduced bymaximally 30%. Experiments have shown that above an alloy dependingthreshold from temperature and holding period a steep drop of theproperties of the cold component occurs wherein in particular the yieldstrength R_(p0.2) is significantly reduced. In particular thetemperature dependent threshold of the process window is very narrow,but also the holding time is increasingly more critical when thetemperature-time threshold is approached. In order for the deformationcapacity to be sufficiently great however, a comparatively hightemperature of up to 300° C., depending on the component geometry, isrequired to achieve the forming, from this depending on the alloy, arelatively small process window can result. Generally, the manufacturingprocess is associated with a slight loss of strength with increasingheating and holding time at the temperatures required for forming sothat according to the invention, overall times at temperature which areas short as possible are desired. Because certain tact times cannot oronly associated with uneconomical effort be fallen below for thehandling after the heating and the pressing of the sheet metals, aheating up which is as fast as possible offers the best possibilities toproduce components with a strength which is close to the strength of thestarting sheet metal. For this, the sheet metal has to be heated to atemperature of at least 200° C. and at most 400° C., preferably 250° C.to 370° C., in particular 270° C. to 300° C. The associated heating timefor the method according to the invention are at 1 to 60 seconds, andwith this significantly below the ones which are possible in aconventional furnabe heating to the mentioned temperatures. The heatingcan therefore occur resistively, conductively, inductively, orcapacitively. Wherein the period of heating as describe above ispreferably shorter and should be below 45 seconds, in particular fortemperatures above 250° C. below 30 seconds.

Preferably, the sheet metal—as described—is heated within a time periodof 1 to 30 seconds and formed within maximally 60 seconds and cooled atleast locally below 260° C.

The European standard EN515 governs the nomenclature of basic states ofaluminum semi finished products. The letter H means strain hardened.This term applies to products which for ensuring the defined mechanicalproperties are subjected after soft annealing (or after the hot forming)to a cold forming or a combination of cold forming and recoveryannealing or stabilizing. The letter H is followed by two digits, thefirst for indicating the type of the thermal treatment, the second forindicating the degree of the strain hardening.

Cold forming relates to the plastic deformation of a metal at atemperature and speed which leads to a strain hardening. Strainhardening is the change of the metal microstructure by cold formingwhich leads to increased strength and hardness, wherein the formabilitydecreases.

The temper designation H1x only designates strain hardened productswhich for achieving the desired strength are strain hardened withoutadditional thermal treatment.

The temper designation H2x means strain hardened and re-annealed. Thisapplies for products which are strain hardened in excess of the desiredfinal strength and in which the strain hardening is decreased to adesired strength by re-annealing.

The temper designation H3x means strain hardened and stabilized andapplies to strain hardened products whose mechanical properties arestabilized by a thermal treatment at low temperature or by a heatingcarried out during processing. Stabilizing improves generally theforming capability. This designation only applies to alloys whichdecrease in strength during storage in the absence of stabilizing.

The second digit after the H indicates the final degree of the strainhardening which is characterized by the minimal values of tensilestrength. The digit 8 is assigned to the hardest states, which areusually produced. The digit 9 indicates states whose minimal tensilestrength exceeds the states H8x by 10 MPa or more. The digits 2, 4 and 6indicate intermediate states. Accordingly, H12 means strain hardened ¼hard, H14 strain hardened-½ hard, H16 strain hardened-¾ hard and H18strain hardened-4/4 hard (fully hardened). Accordingly, H22/24/26/28designate strain hardened and re-annealed materials or respectively thetemper designations H32/34/36/38 designate strain hardened andstabilized materials. Overall, sheet metals are therefore to be usedwhich are roll hardened and with this have been subjected to a strainhardening by rolling.

The invention has the advantage that a relatively complex component canbe produced from a roll hardened non precipitation-hardenable aluminumalloy, which in its entirety or in sub regions has a high strength. Theparticular feature is that the sub-regions of high strength do notdepend on the deformation and the introduced degrees of deformation.This provides an alternative manufacturing path compared toprecipitation-hardenable alloys which due to significantly higher costsof the semi finished products and heat treatment of the component whichlasts up to 24 hours are associated with high production costs.

An open sheet metal molding according to the invention, is a componentproduced from a sheet metal blank by pressing, i.e., from a componentwhich in its starting state is essentially flat. The starting materialwas strain hardened to a defined target value by rolling. An open sheetmetal component according to the invention does not mean a hollowsection.

Within the framework of the invention, the roll hardened nonprecipitation-hardenable aluminum sheet metal is preferably heated inits entirety and formed. However, a partial hot forming is alsoconceivable, in that the sheet metal has regions which are heated fordifferent periods of time, be it to achieve different targettemperatures or to influence the local deformation properties of thesheet metal or the tensile strength by the duration of the heating.

The sheet metal can preferably be heated resistively, conductively,inductively, or capacitively.

After the forming of the sheet metal, portions of the sheet metal or thesheet metal molding can be further held at temperature or heated in adevice.

With the method on which the invention is based, it is also possible toproduce components which are to have a high strength only, in definedregions, wherein the components can have a targeted lower strength inother regions, at simultaneous improved elongation values. This isgeared toward producing a component which is generally improved comparedto an entirely hard component, and which, in particular with regard tothe deformation behavior in case of an accident, has more favorableproperties, be it with regard to energy absorption or with regard toweight saving.

Within the framework of the invention it is also possible to continuethe heating of the sheet metal during the forming process, which heatingdiffers regionally in strength and duration. For this, the forming toolcan have a forming recess with at least one region which is heated. Asan alternative or in addition to heated sites in the tool, cooledregions can also be provided. The heated regions in the forming tooloffer the possibility to hold the work piece within an additional periodof time at a higher temperature by heating a defined region for anextended period of time.

In a multi-step forming, forming recesses can also be provided infurther tool steps, which are partially heated. In the same manner, atemporally extended or higher heating is possible in a process stepfollowing the forming by means of a partially acting furnace or aninductor. It is regarded as useful however, to realize the variantinvolving higher heating temperature prior to the forming and notsubsequent to the forming.

Automobile parts, which currently are preferably manufactured inprecipitation-hardenable alloys and in the future can advantageously bereplaced for motor vehicle sheet metal moldings according to theinvention are:

-   A, B, C—columns-   Longitudinal members (sill, center tunnel)-   Cross members (seat-/roof-)-   Crash boxes-   Bumper-cross members-   Reinforcement sheet metals in general-   Reinforcement sheet metals in the floor region-   Front wall cross members (up, center, down)-   Heel plate cross beams-   Door parapet-   Suspension-strut dome

Automobile parts which are currently produced from soft nonprecipitation-hardenable alloys and can in the future be advantageouslyreplaced for motor vehicle sheet metal moldings according to theinvention are:

-   Door inner panels-   Shear areas-   Floor panels-   Rear hatch inner panel+hood inner panel-   Cross beam (seat-/roof-)-   Reinforcement panel in general-   Reinforcement panel in the floor region-   Front wall cross beam (up, center, down)-   Heel plate cross member-   Longitudinal cross member (sill, center tunnel)

Crash tests have shown that motor vehicle sheet metal moldings producedaccording to the invention have a surprisingly high ductility, thismeans great deformations of the components are possible without failuredue to cracks. In addition, the tests have shown that the energyabsorption in case of a crash at same intrusion is 15% higher than insheet metal moldings made from the same alloy which, however, were notsubjected to the method according to the invention. Even greater is thedifference in energy absorption until the initiation of the plasticdeformation, due to the high R_(p0.2). This is important for componentswhich are intended to conduct high (crash) forces, without plasticallydeforming themselves (for example bumper cross members).

What is claimed is:
 1. A method for producing an open sheet metalmolding comprising the steps of: providing a sheet metal made of a rollhardened, non precipitation-hardenable aluminum alloy comprisingaluminum and magnesium, said sheet metal being in a material statedefined by a temper designation according to European standardEN515:1993 selected from the group consisting of H12, H14, H16, H18,H19, H22, H24, H26, H28, H32, H34, H36 and H38; heating at least aportion of the sheet metal to a temperature in a range from 250° C. to370° C. within a time period of 1 to 60 seconds; inserting the heatedsheet metal into a forming tool of a forming die; forming the sheetmetal into the sheet metal molding; and actively cooling the portion ofthe sheet metal molding to a temperature below 200° C. within 60 secondsafter the heating by placement in a cold forming tool.
 2. The method ofclaim 1, wherein the aluminum alloy further comprises manganese.
 3. Themethod of claim 1, wherein the sheet metal is heated in the heating stepto a temperature in a range from 270° C. to 350° C.
 4. The methodaccording of claim 1, wherein the portion of the sheet metal has regionswhich are heated to different temperatures prior to the forming step. 5.The method of claim 1, wherein the portion of the sheet metal hasregions which are heated for different respective periods of time.
 6. Amethod for producing an open sheet metal molding comprising the stepsof: providing a sheet metal made of a roll hardened, nonprecipitation-hardenable aluminum alloy comprising aluminum andmagnesium, said sheet metal being in a material state defined by atemper designation according to European standard EN515:1993 selectedfrom the group consisting of H12, H14, H16, H18, H19, H22, H24, H26,H28, H32, H34, H36 and H38; heating at least a portion of the sheetmetal to a temperature in a range from 250° C. to 370° C. within a timeperiod of 1 to 30 seconds; inserting the heated sheet metal into aforming tool of a forming die; forming the sheet metal into the sheetmetal molding; and actively cooling the portion of the sheet metalmolding at least locally to a temperature below 200° C. by placement ina cold forming tool, wherein the forming and cooling step are performedwithin 60 seconds after the heating step.